As described e.g. on the Internet site of the University of North Carolina, school of medicine, Whitehead Medical Society, First Aid for the first year, several parameters can be determined in order to describe the respiratory mechanics and gas exchange of a subject's lung:                FVC—forced vital capacity—total amount air exhaled        FEV1—amount of air exhaled during the first second of forced expiration        FRC—functional residual capacity        TLC—total lung capacity        RV—residual volume        IC—inspiratory capacity—amount of air inhaled above FRC; IC=TLC−FRC        ERV—expiratory reserve volume—amount of air exhaled below FRC; ERV=FRC−RV        
FVC, FEV1, IC and ERV may be determined by spirometry. Process begins with patient breathing normally for several cycles. Normal breathing is represented by FRC—the lung volume at which the elastic forces of the lung are equally opposed by the outward pull of the chest wall. Tidal volume is the amount of air inhaled during normal breathing. Then the patient inhales to TLC and exhales forcefully to RV.
FRC may be determined by (whole body) Plethysmography, Helium dilution or Nitrogen washout technique, which are described e. g. in “Respiratory Mechanics in Infants: Physiologic Evaluation in Health and Disease”, by a joint committee of the ATS Assembly on Pediatrics and the ERS Paediatrics Assembly, American Review of Respiratory Disease Vol. 147, 1993, p. 475-496.
The nitrogen washout technique is to measure the volume of nitrogen washed out of the lungs when subject rebreathes from a reservoir of nitrogen-free gas. In the original studies the equipment used was fairly simple yet cumbersome to operate, later real-time techniques relied on mass spectrometers, which are technically demanding to maintain. If the amount of washed out nitrogen is measured and the initial concentration of alveolar nitrogen is known, then the lung volume at which the washout started can be derived. If washout starts at FRC, then FRC equals the volume of nitrogen washed out divided by the initial nitrogen concentration in the lungs.
The difficult aspect of this technique is the accurate measurement of the volume of nitrogen washed out. In the two most commonly used methods, the volume of nitrogen is either measured from the expired gas in a collection bag or obtained by continuous integration of nitrogen concentration in the expired gas.
In the expired gas collection method, the expired nitrogen volume is calculated as the product of the nitrogen concentration and the bag volume. Any inaccuracy in the measurement of the bag volume or, more commonly, the final nitrogen washout concentration, will cause significant errors. Because the final nitrogen concentration is very low, having been diluted with large amounts of oxygen, an error of <1% in its measurement will cause substantial error. The resolution, and thus the accuracy, of the method depends on the initial alveolar nitrogen concentration.
Using rapidly responding gas analyzers (or mass spectrometers) to obtain instantaneous nitrogen concentration and a computer to integrate flow signals, an open circuit system can be created. The expired volume and the associated nitrogen concentration are measured continuously by fast response gas analysers or mass spectrometers. A variation of this technique used a constant bias flow, higher than the peak inspiratory flow. This bias flow diluted the expiratory nitrogen concentration and resulted in similar accuracy problems as the collection bag systems.
Furthermore the large continuous bias flow consumed large amounts of oxygen gas, increasing the cost of the test.
Other potential problems with the nitrogen washout technique include those associated with analyzer response time, lag time between flow rate and gas concentration, and sampling rate.
Corrections must be made for the nitrogen flushed from the tissues and blood. The latter usually causes <5% error within a typical washout period of 2 to 3 minutes but may be larger if washout is prolonged in patients with lung disease. In the usual adult methods, end-tidal nitrogen concentration is measured continuously and required to decrease to <2%, because concentrations higher than 2% tend to overlook the effects of extremely slow spaces. Final nitrogen concentrations of <1% tend to exaggerate the effect of normal nitrogen diffusion from pulmonary blood to the alveolar space. In some methods the final concentration is derived from exponential analysis of only a few breaths.
As with all gas dilution techniques, results may be invalidated by leaks.
Further, it is clinically useful (cf. osy-RespNotes.DOC, I.c.) to measure diffusing capacity for carbon monoxide (DLCO). Lung has a large surface area for gas diffusion of about the size of a tennis court and diffusion barrier is very small (0.1 Mm-1.0 Mm). CO is used because its uptake rate is limited only by diffusion process even in the normal lung. However, patients with low hemoglobin levels will have falsely low DLCO and a correction factor is used. Diseases that affect thickness of diffusing surface (emphysema, interstitial fibrosis) are notable for low DLCO. Diffusion capacity for CO=volume of CO transferred in milliliters/minute per mmHg of alveolar gas pressure DLCO=VCO/PACO; VCO is the rate of CO uptake and a measure difference between known concentrations inhaled and amount exhaled after 10 seconds.
In all washout, dilution or diffusion techniques it is important that subject breathes a gas with well-defined composition and is able to exhale gas for immediate analysis or storage and delayed analysis. The inspiratory gas flow control may be performed by so-called demand valves. Compared to inspiratory bag solutions, they offer reduced gas consumption, less potential for inspiratory gas composition deviations due to leaks and easier handling.
Mechanical embodiments of demand valves are also used for diving or resuscitator equipment. In typical mechanical demand valves, the breathing of the subject causes a pressure difference which bends a membrane. The bent membrane opens a valve to a pressurized air or oxygen reservoir. From the pressure difference and the gas flow through the valve a flow resistance may be calculated. A high flow resistance reduces compliance of subjects with necessary maneuvers for washout, dilution or diffusion techniques.
The pressure drop in a tube can be calculated from the following formula (Technische Strömungsmechanik 1, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig):
                              Δ          ⁢                                          ⁢          p                =                              ξ            ⁢                          ρ              2                        ⁢                          v              a                                =                                                                      λ                  ⁢                                                                          ⁢                  I                                d                            ·                              ρ                2                                      ⁢                          v              a                                                          (        1        )            
Δp thereby is the pressure dropping in the tube, ξ is a pressure loss correction value of the tube, λ is a pipe friction value of the tube, | is the length of the tube, d is the diameter of the tube, ρ is the density of the flowing medium, i.e. approximately 1.2 kg/m3 for air, and v is the flow velocity averaged over the cross-section. a has the value 2 for turbulent flows and 1 for laminar flows. In practice a may also adopt intermediate values, as an ideal-typical form of flow is rare. Equation (1) is also known from Strömungslehre, J. H. Spurk, 4th Edition, Springerverlag, Berlin 1996, wherein λ here is called the flow resistance coefficient. The averaged flow velocity is connected with the air flow {dot over (V)} as follows:{dot over (V)}=v·π·(d/2)2  (2)
V itself stands for an air volume. The point designates the derivative with respect to time d/dt.
If one inserts (2) in (1), one obtains the following quadratic dependence of the pressure drop Δp on {dot over (V)}. The dependencies of λ, |, d and p were combined to the constant C, with C being a parameter for the used tube:Δp=C·{dot over (V)}a  (3){dot over (V)}=C′Δp1/a  (4)
Equation (4) is equivalent to equation (3), but solved for {dot over (V)}.
The WO 98/31282 A1 corresponding to U.S. 2002/0185126 A1 and DE 197 46742 A1, discloses a controlled gas-supply system. Gas sources provide O2, NO, N2He or CO2 under an excess pressure. The flow of each gas source is controlled by a valve, which may in turn be controlled by a pressure sensor close to the nose of a patient. The pressure drop at the beginning of the inspiration may be used as a trigger signal for a valve control.
It is desirable to provide a measuring head and a method for controlling gas flow that combine increased subject's comfort with moderate gas consumption.